Most industrial and automotive lubricants contain an anti-oxidant or a combination of anti-oxidants to extend the lubricants' useful operating life. In some applications, such as automotive engine oils, there is a need for lubricants to perform under high thermal stresses where the lubricant can experience temperatures of for example 250 degrees Celsius (° C.) or higher. In addition, lubricants having longer drain intervals are desired. Currently for example, oil drain intervals for passenger cars using current motor oils require a drain interval every 3 to 4 months over the life time of the automobile. One potential technical solution to extending the life or oil drain intervals of a lubricant is to develop new anti-oxidants useful in lubricants or to develop combination of current commercial anti-oxidants with other materials that provide a synergistic performance in extending oil drain intervals.
Heretofore, the most common types of anti-oxidants used in lubricants are alkylated diphenylamines (ADPA). Another type of anti-oxidants used in lubricants is hindered phenolic anti-oxidants. Both of these types of anti-oxidants are often described as “free radical scavengers”. A second class of anti-oxidants is a “peroxide decomposer”. This second class of anti-oxidants' mode of action is very different to the free radical scavengers. The action of the peroxide decomposers is to reduce the alkyl hydroperoxides to alcohols. These hydroperoxides form from the radical decomposition of the lubricant base oil. In this way peroxide decomposers are consumed in a sacrificial manner. Conventional peroxide decomposers include sulphur-containing organometallic materials such as molybdenum dialkyldithiocarbamates (MoDTC) and zinc dialkyldithiophosphates (ZDDP).
Other antioxidants for use in lubricant compositions have been disclosed heretofore. For example, U.S. Patent Application Publication No. 2014 0213,493 A1 discloses an ashless lubricant composition containing a metal-free sulfur-containing compound including an ashless dithiocarbamate, such as methylene bis(dibutyldithiocarbamate), and sulfurized fatty acids. U.S. Patent Application Publication No. 2014/0045736 A1 discloses lubricating compositions for turbine and hydraulic systems containing an ashless dithiocarbamate anti-wear agent. U.S. Pat. No. 8,093,190 discloses a lubricant anti-oxidant composition containing a metal compound (e.g. molybdenum, tungsten, titanium or boron) and a hindered amine. U.S. Pat. No. 5,994,277 discloses lubricating compositions including copper, a Mo-based compound, an aromatic amine, and a ZDDP. U.S. Pat. No. 6,806,241 B2 discloses lubricating compositions containing an antioxidant additive composition that includes a sulfur compound selected from the group consisting of: (a) thiadiazole; (b) dithiocarbamate; and (c) mixtures thereof. European Patent No. EP 0896 050 A1 discloses lubricant oil compositions including sulphur-based organics such as diester sulphides (e.g. dialkylthioproprionate) as oxidation inhibitors.